Duplex aluminous metal article



Nov. 13, 1962 D. G. VANDENBURGH 3,

DUPLEX ALUMINOUS METAL ARTICLE Filed April 13, 1959 INVENTOR v David 6? Vandenburgh A ORNEY United States Patent fifice attain Patented Nov. 13., 1962 3,063,138 DUPLEX ALUMINOUS METAL ARTICLE David G. Vandenburgh, Lower Burrell, Pa, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 13, 1959, Ser. No. 806,082 3 Claims. (Ci. 29197.5)

This invention relates to protecting an aluminous metal member against corrosion by hot Water of the type used for domestic and industrial purposes.

Aluminum and aluminum-base alloys have been successfully used for storage tanks and pipes for handling water at ordinary temperatures but some alloys have exhibited unexpectedly large amounts of corrosion in systems carrying water at temperatures above 140 F. The hot water appears to alter the surface of these alloys and render them more susceptible to corrosion. According to the electrochemical theory of corrosion some particles in an aluminum base alloy may have a higher electrode potential than the surrounding metal thereby creating a minute galvanic cell, the current flowing from the anodic or high potential component to the surrounding lower potential or cathodic areas in the presence of an electrolyte such as water. Exposure to hot water at temperatures above 140 F. appears to increase the difference in potential between the particles and the matrix. 7

One very effective means of protecting an aluminum base alloy structural member against corrosion is to clad it with a thin layer of a different aluminum alloy which has a higher electrode potential, i.e., one which is anodic to the alloy constituting the structural member. Thus where both portions of the duplex articles are exposed to a corroding medium, the cladding, which is anodic is attacked'preferentially, and is gradually consumed, thereby substantially preventing attack of the cathodic structural or core member. Electrolytic protection of this character is particularly beneficial at the edges of asheet or where the cladding becomes perforated in service and exposes the base or core metal. It has been found, however, that some conventional cladding alloys are less effective in the presence of hot Water than would have been expected from their behavior in contact with Water at room temperature. This failure of the cladding to adequately protect the base or core metal is manifested in these spots where the cladding is perforated and instead of preventing penetration of the base member by lateral enlargement of the opening, as would normally be expected, the pit becomes deeper. Such pitting or penetration of the base or core portion of a hot water pipe or tank Wall is, of course, undesirable since in extreme cases it may Weaken the wall and even result in perforation. The explanation of this behavior is that the cladding fails to prevent the core alloy from becoming anodic in hot water.

The aluminum-manganese type of alloys have been used without any cladding where a high resistance to corrosion combined with moderate strength, and good weldability has been desired. Alloys containing about 1.2% manganese are well-known examples of this type of alloy. Where more severe corrosive conditions are encountered, a cladding of a commercial alloy composed of aluminum and zinc and containing a total of about 0.4 to 0.7% of the usual impurities, such as silicon, iron, copper and manganese, has been used to protect sheet and plate of the foregoing composition. Although tanks and pipes made of such a duplex material have given satisfactory service in many geographical locations for handling domestic or industrial hot water it has been found that they do not have a satisfacor y durability in other localities. In brief, this aluminum-zinc alloy cladding has failed under severe conditions to pro vide the necessary protection against pitting of the aluminum-manganese type alloy base.

It is an object of this invention to provide a duplex aluminous metal article which possesses a superior re sistance to corrosion by hot water under severe conditions.

A particular object is to provide a duplex alumin'ous metal article that resists penetration of the base or core portion of the article when exposed to the action of hot water.

Another particular object is to provide a duplex aluminous metal article having an aluminum-manganese type alloy base that is adequately protected against corrosion by hot water by an aluminum-zinc alloy cladding.

Another specific object of the invention is to provide a material for the construction of hot water tanks and pipes that possesses a very high degree of resistance to corrosion by the hot water.

These and other objects and advantages are achieved by employing a high purity aluminum-zine alloy as a cladding on a base or core member composed of an aluminum-manganese type of alloy. In this combination the aluminum-zinc alloy prevents any reversal of the relative electrode potential of the core alloy in hot water. The superiority of the new duplex article has been demonstrated under extremely adverse conditions such as those encountered where the hot water is acidic and may contain heavy metal salts and deposition of the heavy metal may occur on the aluminous metal surface in contact with the hot water. In referring to hot water, it is to be understood that this means water within the temperature range of about up to or 200 F.

The base or core portion of the duplex article should consist of an aluminum-manganese type alloy consisting essentially of aluminum and 0.5 to 1.5% by weight of manganese. In addition, the alloy may contain 0.4 to 1.5% magnesium for the purpose of increasing the mechanical properties, and also may contain 0.1 to 0.5% copper. The alloy may contain as impurities a maximum of 0.7% iron, 0.6% silicon, 0.25% zinc and the total of all other impurities should not exceed 0.25%.

The cladding or coating layer applied to the foregoing base or core should consist of a high purity aluminum-zinc alloy containing from 0.5 to 6% by weight of zinc, and preferably from 1 to 3% of zinc. In respect to the impurities, the iron should not exceed 0.08%, the silicon should not be more than 0.10% and the copper should not be over 0.02%, with a maximum of 0.01% each for manganese, nickel and chromium and 0.02% for titanium. The total for all of these impurities should under no circumstances exceed 0.15% by weight. It is by reason of these very low limits that the alloy is referred to herein as being of a high purity.

The coating or cladding should be metallurgically bonded to the base metal. This can be accomplished in a known manner with heat and pressure by such operations as rolling, pressing, forging or extrusion. These methods of producing the duplex article result in plastic deformation and create a worked condition as distinguished from a cast or unworked condition, For most purposes the coating should have a thickness of about 2 to 20% of the total thickness or cross section of the duplex article, a thickness of from 5 to being preferred. For the manufacture of tanks, pipes or vessels for handling hot Water, it is usually desirable to employ a duplex material having a thickness of from 0.025 to 0.25" in thickness. The cladding or coating may be employed on either one or two sides of the duplex article depending upon its ultimate use.

The duplex product may be used in the construction of a hot water tank, particularly where magnesium is incorporated in the base alloy. Also the duplex article is particularly applicable in the construction of a flue tube in an internal flue heated hot water tank, as illustrated in FIG. 1, in which the structural requirements in the tube need not be as high as in the shell. In FIG. 1, the tank has a cylindrical vertical wall portion 10 with end walls or caps 16 and 18 welded or otherwise joined to the top and bottom, respectively, of the cylindrical shell, an internal, cylindrical flue tube 11, and a gas burner or the equivalent 13. The cold water inlet tube 24 is suspended in a fitting 22 threaded into a suitable opening or spud in the cap 16 while the shorter tube is provided as the outlet for the hot water, the latter also being threaded or otherwise joined to the cap. The duplex character of the cylindrical wall and top and bottom caps as well as of the cylindrical flue tube is illustrated in the enlarged view seen in FIG. 2 taken on line II-II of FIG. 1. The base or core 12, composed of an aluminum-manganese type alloy, is clad on the inner surface with a high purity aluminum-zinc alloy 14.

The effectiveness of a high purity cladding in protecting the underlying metal as compared to a low purity cladding is illustrated in the following example.

Panels of duplex sheets 0.064" in thickness were fabricated for immersion in hot water. The core or base portion of each panel composed 80% of the total thickness, and the cladding on each surface composed 10% of the total thickness. The core consisted of a commercial alloy having a nominal composition of 1.2% manganese and balance aluminum and impurities. The cladding in one case (A) consisted of aluminum, 1% zinc and 0.4% total of the impurities iron, silicon, copper, etc. and in the other case (B) the alloy was composed of aluminum, 1% zinc and a total of only 0.10% of the same impurities. The duplex sheets having the A and B claddings were immersed in acidic Water (a pH TABLE I Corrosion of Sheet Specimens Exposed to Acidic Water Max. Cladding Penetra- Oladding Depth of Thickness, tion of Pits, Inches Core, Inches Inches It is evident that where the pits in the cladding extended to the core there was considerable penetration of the core in the case of the A cladding but substantially no penetration in the B cladding. Moreover, it was observed that the diameter of the pits was larger in the case of the high purity cladding thus indicating more effective protection.

Having thus described my invention and certain embodiments thereof I claim: I

1. A duplex aluminous metal article comprising a base portion composed of an alloy consisting essentially of aluminum and 0.5 to 1.5% manganese, and a coating bonded to said base that provides electrolytic protection therefor in the presence of hot water, said coating consisting of an alloy of aluminum, 0.5 to 6% zinc and containing not more than 0.15% of all impurities, said article being characterized by a resistance to corrosion by hot water.

2. A duplex aluminous metal article according to claim 1 wherein said base portion also contains 0.4 to 1.5 magnesium.

3. A duplex aluminous metal article according to claim 1 wherein said base portion also contains 0.1 to 0.5% copper.

References Cited in the file of this patent UNITED STATES PATENTS 1,865,089 Dix June 28, 1932 2,428,526 Osterheld Oct. 7, 1947 2,726,436 Champion Dec. 13, 1955 2,874,683 La Rue Feb. 24, 1959 OTHER REFERENCES Gas-Fired Automatic Water Heaters (Consumers Research Bulletin) August 1955, vol. 36, No. 2 (page 23 relied on, Copy in Div. 19, class 122, subclass 17 

1. A DUPLEX ALUMINUM METAL ARTICLE COMPRISING A BASE PORTION COMPOSED OF AN ALLOY CONSISTING ESSENTIALLY OF ALUNINUM AND 0.5 TO 1.5% MANGANESE, AND A 